Device for measuring rotation, associated method and inertial navigation unit

ABSTRACT

A device for measuring rotation including an NMR gyroscope having a sensing axis, a computer, a generating member configured to generate a magnetic field directed along the sensing axis, and a MEMS gyroscope rigidly connected to the NMR gyroscope, the MEMS gyroscope having a sensing axis aligned with the sensing axis of the NMR gyroscope, the MEMS gyroscope being suitable for delivering a MEMS signal representing a rotation about the sensing axis, the computer being configured to calculate, from an NMR signal output by the NMR gyroscope, information relating to a rotation about the sensing axis, and to analyse the MEMS signal over time in order to determine a current cut-off frequency, the computer also being configured to control the generating member in order to generate, over time, a magnetic field of which the amplitude is a function of the current cut-off frequency.

TECHNICAL FIELD

The field of the invention is that of gyroscopes used to enable inertialnavigation, namely navigation in the absence of any external referencepoint by the integration of movement equations. The invention relates toatomic spin gyroscopes that use the magnetic properties of atoms tocarry out rotation measurements, and more particularly those of SEOP(Spin Exchange Optical Pumping) type.

PRIOR ART

The gyroscopes the most widely used at present in inertial navigationare Sagnac effect gyroscopes which have however the drawback of beingrelatively bulky. This is not the case of atomic spin gyroscopes, whichcan be subject to miniaturisation and could be used in numerousinnovative applications, for example to complete GPS data in urbanenvironments intended for drivers or pedestrians, to aid tracking ofemergency services in underground environments, to increase the autonomyof drones in hostile environments, etc.

Atomic gyroscopes use the magnetic properties of atoms (their spin) tocarry out rotation measurements. From the measurement of the evolutionof the magnetic moments of noble gas atoms, it is possible to calculatethe rotation of the gyroscope and thus that of the carrier to which itis attached. This measurement is marred by a certain number ofimperfections, bias drift being the most significant among them. Whenthis drift is of the order of 0.01 degrees/hour, the gyroscope issufficiently precise to serve for inertial navigation, that is to saynavigation independent of any external reference, carried out by doubleintegration of the accelerations measured on the carrier.

To produce hyperpolarised noble gas atoms, atomic spin gyroscopes resortto the SEOP (spin exchange optical pumping) method. This method is basedon the transfer of the angular momentum of photons to the electronicspins of alkali atoms followed by the transfer, by collision, of theangular momentum of said electronic spins of the alkali atoms to thenuclear spins of noble gas atoms.

The first atomic spin gyroscopes developed at the end of the 1960s usednuclear magnetic resonance (NMR). To do so, one or several sensitivespecies contained in a cell are subjected in a continuous manner to astatic magnetic field, which induces a precession of their magneticmoments at a characteristic frequency, called Larmor frequency. Avariation in the value of the Larmor frequency is the sign of rotation,and the magnitude of this variation makes it possible to measure thespeed of rotation of the gyroscope with respect to the inertial frame ofreference.

Another type of atomic spin gyroscope has been developed since the2000s. They are co-magnetometers that are based on a mixture between anoble gas and one or several alkali metals and which, unlike NMRgyroscopes, operate in a regime where the alkali is subjected to amagnetic field very close to zero (potential external magnetic fieldsbeing cancelled by creating opposite compensation fields). Thisefficient architecture is described notably in the article of T. W.Kornack et al. entitled “Nuclear Spin Gyroscope Based on an AtomicComagnetometer”, Phys. Rev. Lett., vol. 95, no. 23, p. 230801, November2005.

However, a major drawback of this device is its start-up time which,physically limited by the slowness of the spin exchange process betweenthe alkali metal (potassium for example) and the noble gas (helium forexample), is of the order of ten or so hours, whereas use in realsituations typically requires a start-up time and a positioning of northof less than five minutes.

This start-up time is linked to the time constant Γ_(ex) ⁻¹ for spinexchange between the alkali metal and the noble gas, because thepolarisation of the latter evolves as:

${P = {P_{alk}\frac{\Gamma_{ex}}{\Gamma_{ex} + \Gamma_{1}}\left( {1 - e^{- {t{({\Gamma_{1} + \Gamma_{ex}})}}}} \right)}},$

where P_(alk) is the polarisation of the alkali metal and Γ₁ therelaxation rate of the noble gas by phenomena other than exchange.

This drawback has been identified notably by the group of Professor Fang(Beihang University, China) who has set up a research programmeconsisting of replacing helium 3 by another noble gas, in thisparticular instance xenon 129, in order to reduce the start-up time tothirty or so minutes, as for example described in the article of J. Fanget al. entitled “A novel Cs-129Xe atomic spin gyroscope with closed-loopFaraday modulation,” Review of Scientific Instruments, vol. 84, no. 8,p. 083108, August 2013. However, the replacement of helium 3 by xenon129 leads to a notable degradation of the performance of the gyroscope.Yet, in order to allow inertial navigation, a drift of the order of0.01°/h and an ARW (angle of random-walk) of the order of 0.002°/√h aretargeted.

DESCRIPTION OF THE INVENTION

The aim of the invention is to reduce the start-up time of an atomicspin gyroscope based on a SEOP type pumping, in order to offer astart-up time compatible with use in real situations of inertialnavigation without however degrading the performance thereof.

It proposes for this purpose a method for detecting rotation of acarrier by means of a device embedded in said carrier and whichcomprises an enclosure containing a gaseous mixture of an alkali metaland a noble gas. The method includes a step of starting up the deviceduring which the noble gas is polarised by means of metastabilityexchange optical pumping. Following the start-up step, the methodincludes a step of acquisition by the device of a signal representativeof said rotation during which the noble gas is maintained polarised bymeans of spin exchange optical pumping.

Certain preferred but non-limiting aspects of this method are thefollowing:

-   -   the start-up step is finished when the polarisation conferred on        the noble gas by means of metastability exchange optical pumping        corresponds to a stationary polarisation conferred on the noble        gas by means of spin exchange optical pumping;    -   the metastability exchange optical pumping includes an        excitation of the noble gas by means of a first pump laser of        which the power is the controlled in such a way that the        polarisation conferred on the noble gas by means of        metastability exchange optical pumping reaches the stationary        polarisation;    -   the start-up step includes a sub-step of polarisation test        including:        -   stopping the metastability exchange optical pumping;        -   starting up the spin exchange optical pumping, carrying out            a first measurement of the polarisation of the noble gas            followed later by carrying out a second measurement of the            polarisation of the noble gas;        -   if the result of the second measurement is greater than the            result of the first measurement, stopping the spin exchange            optical pumping and starting up again the metastability            exchange optical pumping; and        -   if the result of the second measurement is less than the            result of the first measurement, the start-up step is            finished.    -   during the start-up step, the metastability exchange optical        pumping is carried out in an auxiliary cell of the enclosure        connected to a main cell of the enclosure by a diffusion        connection of the gaseous mixture;    -   the start-up step includes:        -   closing a first valve arranged between the main cell and an            intermediate cell arranged in the diffusion connection of            the gaseous mixture; and        -   opening a second valve arranged between the intermediate            cell and the auxiliary cell.

The invention extends to an inertial navigation method implementing themethod for detecting rotation of the carrier.

The invention also relates to a device for detecting rotation,comprising an enclosure containing a gaseous mixture of an alkali metaland a noble gas, and a first system for polarising the noble gasconfigured to carry out spin exchange optical pumping. The devicefurther comprises a second system for polarising the noble gasconfigured to carry out metastability exchange optical pumping, and acontroller configured to implement the start-up and acquisition steps byselectively activating the second and the first polarisation systemrespectively.

The alkali metal may be potassium and the noble gas helium 3.

The enclosure may include a main cell and an auxiliary cell connected tothe main cell by a diffusion connection of the gaseous mixture, thesecond polarisation system being configured to increase the polarisationof the noble gas in the auxiliary cell and the first polarisation systembeing configured to maintain the polarisation of the noble gas in themain cell.

The enclosure may also include an intermediate cell arranged in thediffusion connection of the gaseous mixture, a first valve arrangedbetween the main cell and the intermediate cell and a second valvearranged between the intermediate cell and the auxiliary cell, thecontroller being configured, during the start-up step, to close thefirst valve and open the second valve.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, aims, advantages and characteristics of the inventionwill become clear from reading the detailed description of preferredembodiments of the invention, given by way of non-limiting example, andmade with reference to the appended drawings among which:

FIG. 1 is a flowchart illustrating the main steps of the methodaccording to the invention;

FIG. 2 is a diagram of a cell able to be used in the device according tothe invention.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

The invention relates to a method for detecting rotation of a carrier bymeans of a device embedded in the carrier. The embedded device,typically an atomic spin gyroscope, includes an enclosure containing agaseous mixture of an alkali metal and a noble gas. This device isconfigured to acquire a signal representative of the rotation of thecarrier, and more precisely a signal representative of a shift of theprecession of the noble gas nuclei under the effect of the rotation.

In order to acquire such a signal, the noble gas is maintained polarisedat an equilibrium polarisation by means of spin exchange optical pumping(SEOP). However, in order to reach rapidly the equilibrium polarisationof the SEOP and thus to have available a short start-up time of thedevice, rapid pumping is carried out by another technique, namely theso-called MEOP (Metastability Exchange Optical Pumping) method.According to this method, which does not require resorting to an alkalimetal as pumping intermediate, several noble gas atoms are excited by anelectrical discharge (plasma) to a metastable energy state where theycan absorb light and be optically polarised. Spin exchange then occursbetween the metastable excited state and the fundamental state of thenoble gas. This method is currently used for the production of importantvolumes of hyperpolarised helium for medical imaging applications. Onthe other hand, its use in atomic gyroscopes has not been envisagedbecause in the presence of the plasma numerous undesirable effectsoccur, notably important drifts linked to the interaction of thedifferent species excited by the plasma with those that are used for therotation measurement.

Thus, and with reference to FIG. 1, the invention proposes a method fordetecting rotation of a carrier which includes a step of starting up“DEM-MEOP” the embedded device during which the noble gas is polarisedby means of metastability exchange optical pumping (MEOP) followed by astep of acquisition “MES-SEOP” by the embedded device of a signalrepresentative of said rotation during which the noble gas is maintainedpolarised by means of spin exchange optical pumping (SEOP).

In this method, the start-up “DEM-MEOP” step is typically ended and themeasuring step “MES-SEOP” is begun when the polarisation conferred onthe noble gas during the start-up step by means of the metastabilityexchange optical pumping corresponds to the stationary polarisationconferred on the noble gas by means of a spin exchange optical pumping,namely

$P_{alk}{\frac{\Gamma_{ex}}{\Gamma_{ex} + \Gamma_{1}}.}$

By doing so, transients are avoided during which the measurement wouldnot be optimal.

The invention thus defines a start-up step where rapid polarisation iscarried out by MEOP and a measuring step which does not suffer theimperfections induced by the plasma of MEOP because carried out whilemaintaining stationary polarisation by SEOP.

To do so, the device comprises a first system for polarising the noblegas configured to carry out spin exchange optical pumping and a secondsystem for polarising the noble gas configured to carry outmetastability exchange optical pumping. The device is further equippedwith a controller configured to implement the method of the invention,notably by:

-   -   selectively activating and controlling the second polarisation        system (MEOP) in order to increase the polarisation of the noble        gas during the step of starting up the device; and    -   selectively activating and controlling the first polarisation        system (SEOP) in order to maintain the polarisation of the noble        gas during the step of acquisition of the signal representative        of rotation consecutive to the start-up step.

The enclosure typically contains potassium as alkali metal and helium 3as noble gas.

The first polarisation system SEOP notably includes a first pump laser,a probe laser and a photodetector delivering the signal representativeof rotation of the carrier.

The second polarisation system MEOP includes coils wound on the wall ofthe enclosure which, supplied by a radiofrequency signal, make itpossible to produce inductive coupling of this radio-frequency and fromthere to induce ionisation of the gaseous mixture and thus to generate adischarge plasma capable of populating the metastable state of helium 3(noted 2³S₁). This second polarisation system further includes a secondpump laser capable of emitting an optical beam in the direction of theenclosure to excite the helium 3. This second pump laser is tuned to thetransition between the metastable state 2³S₁ and the excited state 2³Pof helium 3, said transition corresponding to a wavelength of 1083 nm.The controller looks in the power of the second pump laser so that thepolarisation conferred on the noble gas by means of MEOP reaches thestationary polarisation of SEOP. This locking may be carried out bycontinuously measuring the polarisation of the noble gas (for example byusing a coil) and by retroactively acting on the power of the laser ofthe MEOP so that the polarisation of the noble gas at the end of thestart-up step corresponds to the stationary polarisation of SEOP.

The time necessary to reach this stationary polarisation value (durationof the start-up step) depends on numerous parameters (pressure in theenclosure, power of the second probe laser, external magnetic field,etc.). For a typical enclosure, the duration of the start-up step is inthe range 10-300s.

It may not be easy to give a reliable analytical expression of thisstart-up duration and thus to define a priori and in a precise manner atwhat moment to switch from the start-up step to the measuring step. Toovercome this difficulty, it is possible during the start-up step toreiterate a sub-step of polarisation test “TST” to check if thestationary polarisation value has been reached or not. This sub-step mayinclude stopping MEOP, starting SEOP, carrying out of a firstmeasurement of the polarisation of the noble gas followed later, forexample several seconds after, by carrying out a second measurement ofthe polarisation of the noble gas.

If the result of the second measurement is greater than the result ofthe first measurement, the SEOP has increased the polarisation. Thepolarisation had not thus reached its stationary value, and the MEOP hasto be continued. Thus, in such a case, the sub-step of polarisation testincludes stopping the SEOP and starting up again the MEOP.

If the result of the second measurement is less than the result of thefirst measurement, the stationary value is reached, or even exceeded. Insuch a case, the start-up step is finished and the measuring step beginswhile keeping the SEOP going.

The measurements of the polarisation of the noble gas may be carried outby detecting the magnetic field created by the polarisation of the noblegas. To do so, the device may be used as a magnetometer exploiting theresonances of the alkali metal when its optical pumping is carried outin an amplitude modulated magnetic field. Such a procedure is forexample described in Cohen-Tannoudji et al. Revue de Physique Appliquée,vol. 5, no. 1, pp. 102-108, 1970.

In such a case, each of the two measurements of the polarisation takeplace over several characterisation periods, the characterisation periodcorresponding to the square of the product of the targeted stationarypolarisation multiplied by the magnetic moment of helium 3 contained inthe enclosure, divided by the noise of the magnetometer in powerspectral density units.

In an alternative embodiment, the instant of switching between thestart-up and measuring steps cannot be detected by means of measurementsof the polarisation but can be predetermined as being for examplederived from learning based on recordings of switching parameters thatare supplied to a statistical algorithm.

It is known that the regimes for which MEOP are the most efficientcorrespond to high radio-frequency intensities and low helium pressures.

In a possible embodiment represented in FIG. 2, the enclosure includes amain cell 1 and an auxiliary cell 2 connected to the main cell by adiffusion connection of the gaseous mixture 2 and in which a part of thehelium 3 is transferred. The first polarisation system (SEOP) isconfigured to maintain the polarisation of the noble gas in the mainenclosure 1, by means notably of a first probe laser LSEOP arranged insuch a way as to light the main cell 1. The second polarisation system(MEOP) is for its part configured to increase the polarisation of thenoble gas in the auxiliary cell 2, by means notably of a second probelaser MSEOP arranged in such a way as to light the auxiliary cell 2 andcoils (not represented) surrounding the auxiliary cell. Due notably tothe absence of alkali metal on the walls of the auxiliary cell, it ispossible to generate thereon a high intensity plasma which makes itpossible to shorten the duration required to reach the desiredpolarisation level.

In an alternative of this embodiment, an intermediate cell 4 is arrangedin the diffusion connection of the gaseous mixture 3. This intermediatecell 4 has a volume less than that of the auxiliary cell 2, for examplea volume corresponding to 5-10% of that of the main cell.

The enclosure also includes a first valve 5 arranged between the maincell 1 and the intermediate cell 2 and a second valve 6 arranged betweenthe intermediate cell 5 and the auxiliary cell 2. The controller of thedevice is furthermore configured, during the start-up step, to close thefirst valve 5 and open the second valve 3. Thus, by closing theconnection to the main cell and by opening that to the auxiliary cell ofgreater volume, an expansion of the gas takes place which makes itpossible to lower the pressure in the proportion of the respectivevolumes of the intermediate cell and the auxiliary cell to reach theideal pressure regime.

The invention is not limited to the method and to the device describedpreviously but also extends to an inertial navigation unit incorporatingsuch a device, as well as to an inertial navigation method implementedby such a unit and including the carrying out of the method fordetecting rotation of the carrier described previously.

1. A method for detecting rotation of a carrier by a device embedded insaid carrier, said device comprising an enclosure containing a gaseousmixture of an alkali metal and a noble gas, the method including: a stepof starting up the device during which the noble gas is polarised bymeans of metastability exchange optical pumping; and following the stepof starting up, a step of acquiring, by the device, a signalrepresentative of said rotation during which the noble gas is maintainedpolarised by means of spin exchange optical pumping.
 2. The methodaccording to claim 1, wherein the step of starting up is finished whenthe polarisation conferred on the noble gas by means of themetastability exchange optical pumping corresponds to a stationarypolarisation conferred on the noble gas by means of the spin exchangeoptical pumping.
 3. The method according to claim 2, wherein themetastability exchange optical pumping includes an excitation of thenoble gas by means of a first pump laser of which the power iscontrolled in such a way that the polarisation conferred on the noblegas by means of the metastability exchange optical pumping reaches thestationary polarisation.
 4. The method according to claim 2, wherein thestep of starting up includes a sub-step of testing polarisation whichincludes: stopping the metastability exchange optical pumping; startingup the spin exchange optical pumping, carrying out a first measurementof the polarisation of the noble gas followed later by carrying out asecond measurement of the polarisation of the noble gas; if the resultof the second measurement is greater than the result of the firstmeasurement, stopping the spin exchange optical pumping and starting upagain the metastability exchange optical pumping; and if the result ofthe second measurement is less than the result of the first measurement,the step of starting up is finished.
 5. The method according to claim 1,wherein, during the step of starting up, the metastability exchangeoptical pumping is carried out in an auxiliary cell of the enclosureconnected to a main cell of said enclosure by a diffusion connection ofthe gaseous mixture.
 6. The method according to claim 5, wherein thestep of starting up includes: closing a first valve arranged between themain cell and an intermediate cell (4) arranged in the diffusionconnection of the gaseous mixture; and opening a second valve arrangedbetween the intermediate cell and the auxiliary cell.
 7. A device fordetecting rotation, comprising an enclosure containing a gaseous mixtureof an alkali metal and a noble gas, a first system for polarising thenoble gas configured to carry out spin exchange optical pumping, and asecond system for polarising the noble gas configured to carry outmetastability exchange optical pumping, and a controller configured to:during a step of starting up the device, selectively activating andcontrolling the second polarisation system in order to increase thepolarisation of the noble gas; and during a step, consecutive to thestep of starting up, of acquiring, by the device, a signalrepresentative of said rotation selectively activating and controllingthe first polarisation system in order to maintain the polarisation ofthe noble gas.
 8. The device according to claim 7, wherein the alkalimetal is potassium and the noble gas is helium
 3. 9. The deviceaccording to claim 7, wherein the enclosure includes a main cell and anauxiliary cell connected to the main cell by a diffusion connection ofthe gaseous mixture, and wherein the second polarisation system isconfigured to increase the polarisation of the noble gas in theauxiliary enclosure and the first polarisation system is configured tomaintain the polarisation of the noble gas in the main enclosure. 10.The device according to claim 9, further including an intermediate cellarranged in the diffusion connection of the gaseous mixture, a firstvalve arranged between the main cell and the intermediate cell and asecond valve arranged between the intermediate cell and the auxiliarycell, and wherein the controller is further configured, during thestart-up step, to close the first valve and open the second valve. 11.An inertial navigation unit including a device according to claim
 7. 12.An inertial navigation method including the detection of rotation of acarrier in accordance with the method according to claim 1.